The present invention relates to a semiconductor device, more specifically a semiconductor device whose gate turn-on voltage is high.
In terminals of communication movable body, such as portable telephones, etc. MESFETs using GaAs, which are capable of a high-speed operation, are used as parts of power amplifiers of the transmission units.
In the conventional MESFETs; however, a threshold voltage is lower than 0 V, and even with a gate voltage set at 0 V, drain current is not completely turned off, and a trace of drain leak current of tenth-order .mu.A flows. Even a trace of drain leak current is a cause for short battery life for portable telephones, which are long in waiting state without communication (waiting time).
The drain leak current of the MESFET can be made null by applying a high negative voltage to the gate electrode, but to this end separate batteries for generating a negative voltage is necessary. Mounting a plurality of batteries causes volume increase and cost increase unpreferably in terms of properties of the movable body terminal.
In place of mounting extra batteries for generating the negative voltage, it can be proposed that a DC/DC converter is mounted for generating the negative voltage, but this is not preferable because more current than the drain leak current is consumed by the DC/DC converter.
Then, it is proposed that a switch transistor which operates at a positive voltage is inserted in the drain terminal of the MESFET, whereby the drain current is shut off.
However, when the switch transistor operates, and a current flows between the drain and the source, a voltage decrease takes place between the drain and the source, and the voltage decrease becomes larger as the flowing current is increased. Accordingly, a voltage to be applied to the drain electrode of the MESFET adversely lowers. To maintain the same output electric power as in the prior art, a voltage decrease must be supplied by increasing current. In addition, to decrease electric power consumption of the terminal of the communication movable body, the electric power source voltage is lowered, which makes the voltage decrease of the switch transistor more serious.
Under these circumstances a semiconductor device including the barrier layer of AlGaAs layer which is an indirect transition semiconductor layer to increase a threshold voltage to 0.5 V is proposed (Elizabeth Glass, Jon Abrokwah, Rudy Lucero, Eddie Spears, Jeff Rollman, Jenn-Hwa Huang, Bruce Bernhardt and Bill Ooms, "A HIGH EFFICIENCY COMPLEMENTARY GaAs POWER FET TECHNOLOGY FOR SINGLE SUPPLY PORTABLE APPLICATIONS," 1996 IEEE MTT-S Digest, pp. 1083-1086, 1996).
In the proposed semiconductor device the threshold voltage can be raised to about 0.5 V, which makes it possible to turn off the drain current without applying the negative voltage to the gate electrode.
However, in the proposed semiconductor device described above, when a gate turn on voltage, i.e., a gate voltage is set on increase, a voltage at which leak current starts to flow from the gate to the source is as low as 1.6 V. Accordingly, the drain current must be controlled in a 0.5-1.6 V gate voltage range, which makes the device inapplicable to various uses.